Offset printing presses can produce several hundred sheets of printed material per minute. In most cases, the ink is still tacky when the printed sheets come from the printing press so that when the sheets are stacked together the ink from each of the printed sheets may be partially transferred to or it may adhere to the backside of an adjacent sheet. Thus, powder is often sprayed onto the sheets prior to stacking the sheets to prevent the transfer of ink. The powder also facilitates movement of the sheets by an operator by reducing the friction between the stacked sheets.
Laverick, U.S. Pat. No. 4,622,896, discloses a powder spray gun attachment for a sheet delivery tray of a printing press.
Schmoeger, U.S. Pat. No. 4,332,198, discloses an air assist powdering system having a solenoid valve for actuating the powdering system.